1. Field of the Invention
The invention relates to rotary cutting tools and end mills, and more particularly to helically fluted end mills having a plurality of cutting edges unequally spaced with respect to the longitudinal axis of rotation with varying cutting edge geometries.
2. Prior Art
Rotary cutting tools typically employ multiple flutes spaced circumferencially around the tool body and running along the cutting length of the tool. The number of flutes may vary and can run either parallel to the rotational axis, or more commonly, in a helix that is formed as the tool is advanced into a workpiece. Fluted rotary cutting tools have been used for years to perform intricate cutting and sophisticated machining and finishing operations.
Typically, the cutting end includes one or more peripheral cutting edges that are equally spaced with respect to each other having the same helix angle and thus forming a symmetry with respect to the axis of rotation. Moreover, the entire portion of each cutting edge is curved so that the cutting edge lies on the surface of a circumscribing cylinder defined by the rotation of the cutting edge about the rotational axis of the tool body.
Such conventional rotary cutting tools perform adequately at slow rotational speeds. However, at high cutting speeds that are unquestionably preferred, considerable machining difficulty is experienced including forced work stoppage. Such limitations and difficulties stem from the unfortunate development of vibratory forces, more particularly resonant vibration, at higher cutting speeds when the cutting tool begins to chatter resulting in a lack of control over machining and finishing. This can eventually lead to an unsatisfactory work product. In addition, the resonant vibration can reduce the tool life.
Chatter and resonant vibration are problems encountered generally at high speed machining with conventional rotary cutting tools. Thus far, the dynamic stability of the rotary cutting tool has been a limiting factor even though the newer generation of cutting tools generally provide a better cutting performance than their predecessors since many of them now contain a specific coating and are made of a stronger metal or alloy. For a conventional rotary cutting tool, there indeed exists a critical set of rotational speed and depth of cut at which the tool becomes unstable and starts to chatter with sufficient amplitude. This forces an unplanned and undesirable compromise involving the quality of machining, or the life of the tool, or both.
Numerous designs have been suggested in search of improved cutting speeds, improved surface finishing and improved tool longevity while keeping the vibration and the resulting chatter low. These designs have focused on the geometry of the cutting edges, formation of multiple flutes and their configuration in connection with the support structure. The goals have included not only the tool performance along with speed and smoothness of the dampening or chattering, but also prolonging of the tool life.
As indicated earlier, conventional designs for rotary cutting tools include multiple flutes spaced circumferencially around the tool body. They include multiple adjacent cutting blades or cutting edges, each of which is equally spaced about the circumference of the tool body as well. In conventional rotary cutting tools, the cutting edges are designed to be equally spaced in order to allow even loading on the cutting blades. For these reasons, the flutes of conventional rotary cutting tools are of the same size which allows equal load distribution among the flutes. This, in turn, makes a conventionally designed rotary cutting tool strength-limited because of the resonant vibrations caused by such equalizing efforts. As used herein and in the appended claims, the term “circumferencial index” is employed here to indicate angular position about the rotational axis of the tool and it does not suggest or teach a particular space of the tool, or any particular cross-section thereof.
The prior art includes numerous rotary cutting tool designs and improvements. For example, in U.S. Pat. No. 4,963,059 the invention relates to a rotating cutting tool having a plurality of helical peripheral cutting edges with unequal helix angles formed on a circumferencial surface of a tool body. In this invention, at least one of the peripheral cutting edges has a helix angle different from helix angles of the other peripheral cutting edges, but two peripheral cutting edges of such parts of cutting edges share the same helix angles and are symmetrical with respect to the axis of the body. Additionally, the cutting edges are equally spaced circumferencially on the body in at least one plane perpendicular to the axis of rotation of the body.
U.S. Pat. No. 5,810,517 discloses three complexly configured, equally spaced cutting edges. In the first embodiment, the cutting edges are defined by the intersections of right circular cylinders, offset from the axis of the countersink, with the frustum of a cone. In the second embodiment, the cutter body includes a circumferencially, concavely radiused edge defining a quarter section of a torus disposed coaxially about the cutter axis and the cutting edges are defined by the intersection of the flutes. The third embodiment is similar to the first embodiment, except the oblique conical surfaces converge to a vertex. In all three embodiments, it is further disclosed, the cutting edge is a complex arcuate or crescent-shaped curve.
U.S. Pat. No. 5,984,592 shows a rotary cutting tool consisting of a plurality of cutting inserts and containing at least one side insert with a cutting edge projected radically from the peripheral region of the tool body and at least one end insert projected from a forward axial end region of the tool body. These two inserts, it is disclosed, are located in positions that are angularly spaced about a central rotary axis of the tool body and describe cutting envelopes that intersect or overlap in the rotation of the body about said axis. It further teaches that each side insert is secured by clamping screws substantially radially into the tool body and at least two of each said inserts are preferably symmetrically spaced about the central cutting axis of the tool body and in corresponding axial and radial positions.
U.S. Pat. No. 6,065,905 discloses a rotary cutting tool that includes a coating on its radial relief surfaces in order to enhance damping of vibratory motion of the tool at speeds which permit the relief surfaces to rub on the workpiece. It further teaches that in a preferred embodiment, the tool is made of tungsten carbide, steel, or cermet.
U.S. Pat. No. 6,132,146 teaches a rotary cutting tool with a longitudinal axis of rotation, having a cutting head formed with at least two chip evacuation flutes and at least two body portions bearing cutting inserts there between. It also discloses that the operative cutting edge of the second outer cutting insert is substantially shorter than the operative cutting edge of the first outer cutting insert and their outermost ends are substantially equidistant from the axis of rotation.
U.S. Pat. No. 6,152,657 discloses a center cutting end mill wherein flutes extend from the cutting end to the base end and the cutting end having one or more spiral groves and flutes in the walls of a mill body. It also teaches us positioning of a groove adjacent to a leading edge of each flute where each groove is filled with polycrystalline diamond-like material situated in sites within such groove. It also discloses that the diamond-like material is exposed to form a cutting edge along the leading edge of the material extending sufficiently close to the center of the end mill for cutting the workpiece all the way to the center of the end mill.
U.S. Pat. No. 6,382,888 teaches us a vibration dampened spindle and tool holder assembly for a rotary cutting machine, such tool holder having an interfacing ledge with a top surface for abutment with a distal spindle surface and a continuous channel disposed in a proximal portion of the top surface. It further discloses that a resilient dampening member, preferably fabricated from a natural or synthetic rubber composition and having a rectangular or a circular cross-sectional configuration, resides in the channel for compressed abutment with the spindle surface.
U.S. patent application Ser. No. 20020090273 discloses a roughing and finishing rotary tool apparatus and method where said tool preferably has a roughing flute adjacent to each roughing blade and a finishing flute adjacent to each finishing blade. It further teaches that in highly preferred embodiments, the finishing flutes are smaller than roughing flutes, and preferably, each finishing flute is located closer to the preceding roughing flute so that flutes are preferably unequally spaced. In some highly preferred embodiments, according to its teaching, the blades are unequally circumferencially spaced and are immediately behind the flutes. It further discloses that in the most preferred embodiment, all of the finishing blades extend radially farther than all of the roughing blades.
A printed publication by SGS Tool Company discloses a patented Z-Carb milling which reportedly incorporates the design revealed in U.S. Pat. No. 4,963,059, described earlier.
Another printed publication, in the form of a sales literature for VariMill and reportedly based on an invention by Hanita for which patent is pending, discloses a controlled relief in a tool body with helical flutes. The disclosure teaches us that while the helix angles are all the same, the indexes are different. This reportedly makes one set of cutting edges indexed unequally from that of another set of cutting edges.
The foregoing review of patents, published patent applications and miscellaneous printed publications generally reveals diverse designs and cutting configurations of rotary cutting tools and end mills, each attempting to provide an improved performance. The review confirms that additional improvements are not distant goals, but may be achievable now.
The problems associated with resonant vibrations that result in shock resistance have not been solved by the prior art. Such vibrations along with heat and wear resistances are detrimental to a long tool life. While new coating technologies have addressed successfully the issues of heat and wear resistances by introducing a battery of metal, ceramic and chemical substrates, the concern involving resonant vibrations or shock resistance has not been addressed by the prior art. The present invention solves such problems and offers one or more additional improvements over the prior art.